The present invention relates to a power driven ski-bob which is a newcomer to the field of winter recreation.
Presently, the winter sport enthusiasts are polarized. Some adhere to skis propelled by muscles (cross-country) or gravity (Alpine) and some prefer to mount a heavy powerful vehicle, the snowmoble.
A power driven ski-bob, such as shown in my U.S. Pat. Nos. 4,109,739, 4,129,193 and in my U.S. patent application Ser. No. 872,688, all of which are hereby being incorporated by reference, bridge the gap by providing a light weight, agile device which relies on the rider's sense of balance and which uses a small fraction of the power required by a snowmobile.
To achieve this unique effectiveness of design (the power driven ski-bob may weigh one tenth of the the average snowmobile), the power driven ski-bob relies on a plurality of subtle design features, some of which are described in my previously referred to patents and patent application as well as on the improvement shown herein.
A power driven ski-bob develops traction through its propulsion unit which preferably comprises an anterior sliding surface and a posterior endless tread portion. For example, a power driven ski shown in my U.S. Pat. No. Re. 29,657, which is herein being incorporated by reference, would make an efficient propulsion unit. The efficiency of the propulsion unit can be further improved, dramatically, by biasing weight between the anterior and posterior portions and by adjusting the angle of attack between the propulsion unit and the snow, according to the snow conditions (deep, powdery, etc.) and to the ride mode (acceleration, coasting, going up or down hill, etc.). Generally, it can be said that under normal conditions, it is preferred to distribute a substantial amount of the power driven ski-bob's gross weight (gross weight includes the rider's weight), on the sliding surfaces since they develop the least amount of drag as a result thereof, and since it does not contribute to internal friction in the endless tread portion and associated tread wear as does weight which is born directly by the endless tread portion. However, when increased traction is required for accelerating and/or climbing a hill, it is important to shift weight to the endless tread portion to assure traction and secure the engagement of the tread with the snow. Further, in deep heavy snow, when the going is rough, the propulsion unit is sunken and countinously tries to get out of a hole (which becomes an elongated channel as the propulsion unit moves forward). Under this condition the propulsion unit should be inclined to increase the angle of attack between its undersurface and the snow in its path (as in an airplane's wing before landing). Providing such a suitable geometry for this condition substantially improves the ability of the power driven ski-bob to move in deep heavy snow.